U.S. Pat. No. 5,727,492 discloses an example of a semi-membrane tank capable of holding liquefied gases such as liquefied natural gas (“LNG”). Semi-membrane tanks for this purpose, however, lack sufficient wall strength and rigidity to be self-supporting and use a supporting structure to transfer load from the semi-membrane walls to surrounding structure. The supporting structure typically includes a grid of beams connected to the semi-membrane walls and connected to an arrangement of support assemblies connecting the grid of beams to a surrounding structure. When a semi-membrane tank is installed in a ship or other containment structure, the structure of the ship may serve as the surrounding structure connecting to the support assemblies. Due to the low temperatures required to transport or store liquefied gases, insulating material or support blocks with low thermal conductivity are typically used to thermally isolate the semi-membrane tank from the surrounding structure.
The temperatures experienced by a semi-membrane tank fluctuate dramatically between ambient temperature and the very low temperatures of liquefied gases, such as −161 degrees Celsius for LNG. Depending on the coefficient of thermal expansion (“CTE”) of the semi-membrane tank material, this temperature fluctuation results in thermal contractions when the semi-membrane tank cools. If the semi-membrane tank is rigidly attached to the surrounding structure, the thermal contractions and expansions during temperature fluctuations of the tank may induce unacceptable stresses on the surrounding structure. To alleviate this condition, the support assemblies connecting the semi-membrane tank and the surrounding structure may be configured to allow for relative movement between the semi-membrane tank and the surrounding structure.
U.S. Pat. No. 6,971,537 shows a support arrangement for a semi-membrane LNG tank having a support assembly allowing relative motion between the semi-membrane tank wall and the surrounding structure in two orthogonal directions. More particularly, the support assemblies include a complicated set of grooves and brackets that receive a connecting element in a linear sliding motion. The support arrangement provides for linear sliding motion in a groove in a first direction and linear sliding motion in a bracket in another direction. Although the horizontal center line of the LNG tank disclosed in the U.S. Pat. No. 6,971,537 may be supported in the vertical direction, the support assemblies disclosed allow the tank to expand and contract in the vertical and horizontal directions using the linear sliding configuration of the support assemblies.
The support assemblies of U.S. Pat. No. 6,971,537 are imbedded in the insulation covering the semi-membrane tank and requires a flexible boot to be installed around each support assembly. The flexible boot requires inspection and maintenance throughout the life of the semi-membrane tank, which can be costly because inspection typically requires the removal and replacement of tank insulation. Additionally, because the support assemblies only slide linearly along orthogonal directions, each row of support assemblies need to be installed along different radial paths from the center of the tank in order to achieve in-plane unrestricted movement. The configuration of support blocks on the semi-membrane tanks connecting to the support assemblies also complicates the manufacturing of and installation of the complex supporting assemblies. Because the support assemblies slide relative to the supporting blocks, an insulation gap around the support block is necessary to allow for free movement of the support assemblies during thermal expansion and contraction. This may function to increase heat loss to the liquefied gas containment system and increase the boil-off rate. Additionally since the support blocks are required to transfer the thermal and dynamic loads from the tank to the surrounding structure, they may become quite large complicating handling and installation and increasing installation/erection costs.